1. Field of the Invention
This invention relates to powder metallurgically formed steels, and particularly to such steels having enhanced corrosion resistance, and more particularly to h-BN (hexagonal boron nitride) additions to such steels to accomplish enhanced corrosion resistance as well as increased hardness, tensile strength, free machining properties, tightness and surface density. In particular, stainless steels of both austenitic and ferritic type are especially suitable for being produced using a method according to the invention. Powder metallurgy will be referred to as P/M henceforth.
2. Description of the Prior Art
A sintered stainless steel is known where an addition of boron is made to improve the corrosion resistance and the mechanical properties, for example from U.S. Pat. No. 4,032,336 (Reen) which is hereby incorporated as reference. Improved corrosion resistance and improved mechanical properties are due to increase in density. The boron forms a liquid phase during sintering, depleting chromium and molybdenum from the steel powder. The steel powder therefore contains sufficient amount of Cr and Mo to offset this depletion which results in the sintered non-melted parts of the product being within the required composition for a specific austenitic stainless steel. Boron is added to the base material to obtain a pre-alloyed metallic powder which (according to the ASTM handbook Volume 7 p.9) is a metallic powder composed of two or more elements that are alloyed during the powder manufacturing process, and in which the particles are of the same nominal composition throughout.
The raw material thus contains an elevated amount of Cr and Mo, which adds to the cost of the raw material.
According to JP 01-129903 (Wataru), of which the JAPIO English abstract is hereby incorporated by reference, hexagonal boron nitride (h-BN) is mixed with a metallic powder (preferably an iron alloy containing Co, Ni, Cr, etc.). The purpose of adding h-BN to the metal powder is to enable compaction without using an organic lubricating agent, thus utilizing h-BN as a lubricating agent.
It is an object of the invention to provide sintered steels and a method for making steels which contain standard or lower than standard amounts of alloying elements such as Cr, Mo and Ni, but which still exhibit a superior resistance to corrosion as well as increased hardness, tensile strength, free machining properties, tightness and surface density.
In the invention, a steel powder of the desired composition is either directly mixed with a h-BN powder, compressed and then sintered or the steel powder is compressed, impregnated with a solution containing h-BN and then sintered or the steel powder is compressed, sintered and then impregnated with a solution containing h-BN.
Alternatively, steel body formation may be performed by injection molding steel powder in molds (metal injection molding, MIM, also known as powder injection molding). Thus, a steel powder of the desired composition is either directly mixed with a h-BN powder, metal injection molded and then sintered or the steel powder is metal injection molded, impregnated with a solution containing h-BN and then sintered or the steel powder is metal injection molded, pre-sintered, impregnated with a solution containing h-BN and then sintered or, alternatively, the steel powder is metal injection molded, sintered and then impregnated with a solution containing h-BN.
A first method of producing sintered steel bodies according to the invention comprises the steps of:
a) Adding h-BN powder to, and mixing with, a steel powder, preferably a stainless steel powder, in the weight percentage range 0.1 to 2%, more preferably 0.7 to 1%.
b) Compacting the mixed steel powder/h-BN powder using a pressure, preferably in the range of 20-60 tsi, to form green bodies. The unit tsi is converted to MPa by multiplying with 13.793 (or 2000/145), thus the pressure range is approximately 276-828 MPa.
c) Sintering the green bodies to produce sintered steel bodies, preferably at a sintering temperature range of 2000xc2x0 F. (1093xc2x0 C.)-2500xc2x0 F. (1371xc2x0 C.) and for a time of between 15-60 minutes. The sintering step is preferably performed in an atmosphere comprising a mixture of hydrogen and nitrogen.
Alternatively, step b) above may be performed using metal injection molding techniques:
a) Adding h-BN powder to, and mixing with, a steel powder, preferably a stainless steel powder, in the weight percentage range 0.1 to 2%, more preferably 0.7 to 1%, together with a binding mixture, preferably an organic binding mixture.
b) Compacting the mixed steel powder/h-BN powder using MIM.
c) Removing the binder from the green bodies, for example by heating to vaporize the binder.
d) Sintering the green bodies to produce sintered steel bodies, preferably at a sintering temperature range of 2000xc2x0 F. (1093xc2x0 C.)-2500xc2x0 F. (1371xc2x0 C.) and for a time of between 15-60 minutes. The sintering step is preferably performed in an atmosphere comprising a mixture of hydrogen and nitrogen.
A second method of producing sintered steel bodies according to the invention comprises the steps of:
a) Compacting steel powder, preferably a stainless steel powder, using a pressure, preferably in the range of 20-60 tsi (276-828 MPa), to form green bodies.
b) Impregnating the green bodies with a solution containing h-BN.
c) Sintering the impregnated green bodies, preferably at a sintering temperature range of 2000xc2x0 F. (1093xc2x0 C.)-2500xc2x0 F. (1371xc2x0 C.) and for a time of between 15-60 minutes. The sintering step is preferably performed in an atmosphere comprising a mixture of hydrogen and nitrogen.
Alternatively, step a) above may be performed using metal injection molding techniques:
a) Compacting steel powder, preferably a stainless steel powder, together with a binder, preferably an organic binder, using MIM, and removing the binder from the green bodies, for example by pre-sintering heating to vaporize/remove the binder.
b) Impregnating the green/pre-sintered bodies with a solution containing h-BN.
c) Sintering the impregnated green/pre-sintered bodies, preferably at a sintering temperature range of 2000xc2x0 F. (1093xc2x0 C.)-2500xc2x0 F. (1371xc2x0 C.) and for a time of between 15-60 minutes. The sintering step is preferably performed in an atmosphere comprising a mixture of hydrogen and nitrogen.
If the binder is left in the green bodies during the impregnating step, the binder will be removed by the heat during the sintering step. This is useful only for binders which do not impede the impregnating step or adversely affect the impregnation effect during sintering. An optional step of pre-sintering the green bodies is preferably performed between steps a) and b) above.
A third method of producing sintered steel bodies according to the invention comprises the steps of:
a) Compacting steel powder, preferably a stainless steel powder, using a pressure, preferably in the range of 20-60 tsi (276-828 MPa), to form green bodies.
b) Sintering the green bodies, preferably at a sintering temperature range of 2000xc2x0 F. (1093xc2x0 C.)-2500xc2x0 F. (1371xc2x0 C.) and for a time of between 15-60 minutes. The sintering step is preferably performed in an atmosphere comprising a mixture of hydrogen and nitrogen.
c) Impregnating the sintered bodies with a solution containing h-BN.
Alternatively, step a) above may be performed using metal injection molding techniques:
a) Compacting steel powder, preferably a stainless steel powder, together with a binder, preferably an organic binder, using MIM, and optionally removing the binder from the green bodies, for example by heating to vaporize the binder.
b) Sintering the green bodies, preferably at a sintering temperature range of 2000xc2x0 F. (1093xc2x0 C.)-2500xc2x0 F. (1371xc2x0 C.) and for a time of between 15-60 minutes. The sintering step is preferably performed in an atmosphere comprising a mixture of hydrogen and nitrogen.
c) Impregnating the sintered bodies with a solution containing h-BN.
The product of the method according to the invention is thus a sintered steel, preferably a stainless steel, having a composition of essentially iron, and possible alloying elements such as chromium, molybdenum and nickel, together with 0.1 to 2% h-BN, preferably 0.7 to 1% h-BN.
Further features of the invention will be described or will become apparent in the course of the following detailed description.